This invention relates to a key telephone system using a SIP (Session Initiation Protocol) and, in particular, to a method of receiving and answering an incoming call for a representative group in the key telephone system.
As well known in the art, a key telephone system comprises a main unit connected to at least one station line (or at least one external telephone line) and a plurality of key telephones each of which is connected to the main unit via a communication line and a data line (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. H3-74996 (Patent Document 1)).
In recent years, the key telephone system is developed so as to support or integrate VoIP (Voice over Internet Protocol). Herein, VoIP is a technique of transferring voice data on IP (Internet Protocol) which originally is a protocol for data communication, and is intended to achieve Internet telephony. As a typical call control protocol of VoIP, attention is focused upon SIP (Session Initiation Protocol).
Herein, SIP is an application layer signaling protocol for starting, modifying, and terminating one-to-one communication or collaborative communication among a plurality of participants, such as telephony and instant messenger, and is defined by RFC (request for comments) 3261. The signaling protocol covers, in case of telephony, a part of calling a called party and a part of being called by a calling party and controls the start to the end of communication.
In the present specification, a key telephone having a SIP function (i.e., a telephone supporting SIP) is called a “SIP telephone”.
In a functional aspect, an existing key telephone system accommodating an existing key telephone (multifunctional telephone) provides various functional services and is enriched. In order to promote integration of VoIP into the key telephone system, the functional services provided by the existing key telephone system must be achieved also by a key telephone system using the SIP telephone.
SIP defines basic sequences such as call origination, call reception, call hold, and call transfer. However, SIP does not define any sequence for achieving the services provided by the existing key telephone system.
Proposal has already been made of the key telephone system using SIP. For example, a “telephone communication system” is known which has a simple structure and yet is capable of efficiently executing mutual transmission of telephone information between terminals and of highly flexibly adapting to increase or decrease in number of accommodated terminals (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2002-152224 (Patent Document 2)).
More specifically, Patent Document 2 describes, as an example of a telephone communication system, a key telephone system installed in every home. In order to realize voice packet communication, for example, on an IP (Internet Protocol) network, use is made of SIP (RFC2543), SDP (RFC2327), and RTP (RFC1889)/RTCP (RFC1890) as a voice over IP control scheme. SIP (Session Initiation Protocol) is a protocol defined to control initiation, termination, and modification of a session of interactive communication. By placing an incoming call management server for managing “incoming call information” on the IP network, call receiving functions provided by the existing key telephone system, such as ordinary call reception, proxy response, and group call reception can be realized as an IP key telephone system.
In Patent Document 2, in order to provide services equivalent to those in the existing key telephone system by the use of terminals connected to the IP network and to improve system expandability, use is made of distributed arrangement of four functional blocks including an IP telephone, a SIP server, a resource management server, and an application server. Herein, a combination of the SIP server, the resource management server, and the application server corresponds to a main unit of a key telephone system.
The IP telephone is a LAN terminal compliant with IEEE802.3 and has a SIP user agent function. The SIP server is a LAN terminal compliant with IEEE802.3 and has a SIP server function. The resource management server is a LAN terminal compliant with IEEE802.3 and has a server function of managing, storing, and delivering a terminal status database of terminals belonging to the IP key telephone system. The application server is a LAN terminal compliant with IEEE802.3 and has a server function of providing a call reception service function of the IP key telephone system.
Patent Document 2 discloses, as a call answering sequence, a step of issuing a “Notify: BUSY” message to the resource management server when a call-receiving or called IP terminal during call reception performs a call answering operation such as a handset lifting operation. Further, Patent Document 1 discloses, as a call proxy response sequence, a step of notifying a session initiation request message (INVITE) defined by RFC2543 from a call management server to a proxy response terminal upon performing a call proxy response.
A “group telephone system” is known which enables a non-ringing group phone terminal to answer an incoming call to a shared number by picking even if the shared number is not known (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2005-110099 (Patent Document 3)). In Patent Document 3, in order to answer the incoming call from a non-ringing telephone outside a shared number group, a primary number of a ringing telephone is inputted first and then a pickup key is pressed to perform a pickup operation. In response, the ringing telephone transmits a “NOTIFY” message to the non-ringing telephone. Then, the non-ringing telephone transmits an “INVITE” message for requesting an external line to change connection of the incoming call.
Further, proposal is made of “Transfer Method and Transfer Program in a SIP Terminal” which are capable of executing and terminating normal transaction by a transferring operation in a call reception limited condition (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2005-136564 (Patent Document 4)). In Patent Document 4, a “NOTIFY” message is transmitted in order to notify a status.
Proposal is also made of an “IP Telephone Gateway Apparatus” in which, in case where a plurality of external telephone numbers are acquired and a plurality of telephones are provided, it is possible to divide the telephones into a plurality of call number groups so as to specify which telephones should receive an incoming call addressed to a particular one of the external telephone numbers without delivering an extra or additional packet to an IP network (see Japanese Unexamined Patent Application Publication (JP-A) No. 2005-244490 (Patent Document 5)). According to Patent Document 5, it is possible to switch sequential call reception and simultaneous call reception. In the sequential call reception, call reception requests are sequentially sent to the telephones in a predetermined order. In the simultaneous call reception, call reception requests are simultaneously sent to all of the telephones. Patent Document 5 discloses that, in case where the simultaneous call reception is carried out, an IP telephone gateway device of a called party simultaneously sends ringing requests to a plurality of available telephones and that, if hook-off of any telephone is detected, the IP telephone gateway device of the called party stops ringing of other telephones (unanswering telephones).
In the meanwhile, as one of call reception service functions provided by the existing key telephone system, a representative group call reception service function is known. Herein, the representative group call reception service function is a call reception service function in which a main unit detects a call to a number assigned to a representative group of SIP telephones and then simultaneously calls all of the SIP telephones forming the representative group.
Patent Document 2 may disclose several call reception service functions in the key telephone system using SIP. However, Patent Document 2 neither discloses nor suggests the representative group call reception service function. Thus, Patent Document 2 discloses, among various call reception service functions, no more than an ordinary call reception service function (call answering sequence) and a proxy response call reception service function (call proxy response sequence). No disclosure is made of the representative group call reception service function (group call response sequence).
Patent Document 3 discloses an example of use of a “NOTIFY” message. However, Patent Document 3 only discloses a conventional example of call pickup and neither discloses nor suggests the representative group call reception service function.
Patent Document 4 merely discloses the use of a “NOTIFY” message in case of a transfer service but neither discloses nor suggests the representative group call reception service function.
On the other hand, Patent Document 5 may disclose simultaneous call reception and call answering thereto. However, Patent Document 5 does not disclose specific examples about how to request simultaneous ringing and how to stop ringing of other telephones.